U.S. Pat. No. 3,763,248 discloses the polybromination of aromatic compounds including diphenyl ether (diphenyl oxide) with bromine using certain metal halide halogenation catalysts such as AlCl.sub.3 and AlBr.sub.3 and solvent quantities of methylene bromide. The amount of bromine used is from 4 to about 12 moles of bromine per mole of aromatic compound to be brominated. The amount of methylene bromide is from about 8 to about 20 moles and preferably from 10 to about 15 moles per mole of aromatic compound.
British No. 1,411,524 describes bromination of aromatic compounds including diphenyl oxide by gradual addition of one reactant to the other while maintaining the quantity of bromine throughout the reaction at a minimum excess of 20 percent with respect to the quantity stoichiometrically necessary for the reaction. The process is conducted in the absence of a solvent. Decabromdiphenyl ether was prepared by gradually adding 0.3 moles of molten diphenyl oxide to 5 moles of bromine and 2.5 g of anhydrous aluminum chloride.
To the same general effect is U.S. Pat. No. 3,965,197 which describes production of decabromodiphenyl oxide by adding molten diphenyl oxide (diphenyl ether) to a large stoichiometric excess of liquid bromine containing a halogenation catalyst such as aluminum trihalide (U.S. Pat. No. 3,965,197). Although it is not a solvent, the large excess of bromine acts as a liquid medium in which the product is suspended and from which it can be recovered.
Because of its expense, the excess bromine remaining at the conclusion of the reaction must be recovered for reuse. In order to have a manageable reaction mixture and a satisfactory product yield, U.S. Pat. No. 3,965,197 specifies that the large excess of liquid bromine that is used is at least 100 percent of the stoichiometric amount of bromine required for the complete bromination. Thus for every mole of diphenyl oxide used in the process, the reaction system must contain at least 20 moles of elemental bromine. In U.S. Pat. No. 4,287,373, a similar process is described in which the sole liquid reaction medium is composed of at least a 75 percent excess and not more than a 400 percent excess of the stoichiometric amount. In other words, the liquid reaction medium has 17.5 up to 50 moles of bromine per mole of diphenyl oxide.
In other processes, it has been found possible to effectively produce decabromodiphenyl oxide using a 60-70 percent excess of bromine.
Despite the need to use such large excesses of liquid bromine, U.S. Pat. No. 4,287,373 points out that the use of solvents along with smaller excesses of bromine results in serious difficulties. In the words of the patentee:
"Prior art perbromination processes in general have involved use of an up to about 20% excess of bromine in the presence of various kinds of reaction media and solvents such as ethylene dibromide, carbon tetrachloride, chloroform, methylene bromide, acetylene tetrachloride, and the like depending on the particular aromatic compound to be brominated. Perbromination of aromatic compounds has also been carried out in reaction media such as oleum, concentrated sulfuric acid, fuming sulfuric acid, liquid sulfur dioxide, and the like. PA0 Each of these process approaches has serious disadvantages for perbromination, especially for commercial operations. The use of halogenated organic solvents has disadvantages which include low productivities, undersirably slow reaction rates, and the necessity for recovery of the solvents for recycle. In some cases this technique results in the introduction of small but significant amounts of chlorine into the final product, thus limiting product quality. In addition a number of non-condensed aromatic compounds do not undergo perbromination satisfactorily under conditions which can readily be employed using halogenated organic solvents."